Experiments are proposed that are devoted to understanding the biochemical details of mRNA formation in mammalian cells---synthesis of a nuclear precursor, post-transcriptional cleavage and addition of a 5' cap and 3' poly (A) and RNA:RNA ligation---are proposed. In parallel with the studies on mRNA structure and formation explorations will also be made of the site or sites in the mRNA pathway where regulation is effected. Experiments in both normal cells and cells transformed or infected by cancer viruses are described.